The present invention relates to a color image forming apparatus of the electro-photographic method.
For example, in a color image forming apparatus of the electro-photographic method used in copying machines, printers, etc., normally, primary toner images of different colors are formed on a plurality of photoreceptors, these primary toner images are transferred successively onto an intermediate transfer body by primary image transfer thereby forming a color toner image, and subsequently a secondary image transfer is made of this color toner image onto a transfer medium, and thereafter, the color image is formed by fixing the color toner image on the transfer material by carrying out a fixing operation.
Since the potential on the intermediate transfer body is determined by the amount of toner adhered within a specific area, the electric surface potential of the part of the intermediate transfer body of a color image forming apparatus in which toners of several colors have been superimposed will be larger than the electrostatic surface potential of the area in which toner of only one color is adhered, there is the problem that, when the potential distribution is broad in a color toner image formed in this manner, image defects occur in the obtained color image as a result of various transfer defects occurring in the secondary image transfer process.
Methods have been proposed in order to solve the above problem, such as, for example, carrying out pre-charging operation on the color toner image prior to secondary image transfer thereby applying an electric charge with the same polarity as that of the toner potential to the color toner image before the secondary image transfer process and thus raising the potential of the part of the color toner image at which the potential is excessively low to within a range appropriate for carrying out secondary image transfer (see, for example, Patent Document 1 to Patent Document 4).
However, if only the pre-charging operation is made prior to the secondary image transfer, it is not possible to lower the potential of the part of the color toner image at which the potential is excessively high to within a range appropriate for carrying out the secondary image transfer, and as a result of leakage of electrical charge during the secondary image transfer process caused by the presence of parts of the color image toner at which the potential is excessively high image defects are generated such as streaks of discharges, etc. In order to suppress the generation of such discharge streaks, although it is effective to lower the transfer voltage applied during the secondary image transfer process, reducing the transfer voltage is likely to cause image defects in the obtained image such as uneven density or toner splashes due to the presence of parts in the color toner image at which the potential is excessively high.
Patent document 1: Japanese Patent Application Laid Open No. Hei 10-274892
Patent document 2: Japanese Patent Application Laid Open No. Hei 11-143255
Patent document 3: Japanese Patent Application Laid Open No. Hei 11-352793
Patent document 4: Japanese Patent Application Laid Open No. 2004-117884